High-rate anaerobic treatment of domestic wastewater at ambient operating temperatures: A review on benefits and drawbacks

Abstract

This paper reviews the current knowledge on high-rate anaerobic sewage treatment at ambient operating temperatures while presenting the benefits and drawbacks. Since domestic sewage is reported as the main point-source pollutant on a global scale, its treatment deserves ample research. In most of the cities and towns of some developing countries, wastewater produced in households is still discharged into the nearest water body without subjected any treatment. Therefore, simple, affordable, and effective sewage treatment systems are required. Anaerobic digestion of wastewater is reported as a sustainable alternative as recovery of energy is provided while nutrients are preserved for reuse. Anaerobic sewage treatment is certainly not limited to regions of hot climates but it also offers treatment potential in sub-tropical and even in moderate climatic regions due to their favorable temperature conditions. Since many sewage-like dilute wastewaters are discharged at low ambient temperatures especially under moderate climate conditions (15 to 20°C), heating the wastewater to maintain mesophilic conditions (35°C) for anaerobic treatment necessitates high energy and certainly high operating costs. Thus, the development of anaerobic treatment systems operated at ambient temperatures without doubt will have a great ecological and economic impact. High-rate anaerobic treatment systems, especially upflow anaerobic sludge blanket (UASB) reactors, have been occupying a noticeable position for sewage treatment in several tropical countries where artificial heating can be eliminated. However, in spite of their undeniable advantages, there are certain drawbacks of anaerobic sewage treatment at low operating temperatures which should be clarified. Among them, the main concern for anaerobic treatment application is its producing effluents that barely comply with the standarts for reuse in agriculture or discharge to the environment. Therefore, the effluents from anaerobic reactors usually necessitate a post-treatment step as a mean to protect the receiving water bodies. However, anaerobic sewage treatment when combined with a proper post-treatment, provides a sustainable, appropriate, and affordable method especially for developing countries.